1. Field of the Invention
The present invention relates to an electrophotographic processing-based image forming apparatus for printers, facsimile machines, copying machines, etc., and particularly to the image forming apparatus using a contact charging device for charging an electrostatic latent image carrier.
2. Description of the Prior Art
A conventional electrophotographic image forming apparatus is equipped with a charging device for evenly charging the surface of the electrostatic latent image carrier such as a photoconductor drum. The charging device includes a corona discharging device, roller charging device, brush charging device, etc., of which the brush charging device is preferably used from the point of view of downsizing and avoiding generation of ozone.
The brush charging device is the contact charging device which is designed so that a brush of low-resistance bristles comes into contact with the surface of the electrostatic latent image carrier, and a high charging voltage is applied to the brush of bristles via a conductor. After charging the surface of the electrostatic latent image carrier, an electrostatic latent image is formed on the surface by a laser beam, and then, a toner image is formed by a developing device.
In the developing devices, however, there are generated fragments of the toner and/or shavings of the electrostatic latent image carrier which have been charged opposite to the toner charged with intended charge characteristics. Since the polarity of the toner fragments and the shavings is also opposite to the electrostatic latent image carrier, they adhere to the surface of the electrostatic latent image carrier during development. To make the matter worse, they cannot be satisfactorily removed by a cleaning device. As a result, the toner fragments and shavings which have failed to be cleaned off, adhere to the brush charging device and prevent normal charging.
A solution for overcoming this problem is presented in Japanese Patent Application Kohkai (Disclosure) HEI 4-371972 (a Japanese version of U.S. Pat. No. 5,371,578) issued on Dec. 24, 1992, which teaches applying a predetermined voltage having the same polarity as that of the electrostatic latent image carrier and a lower absolute value than the normal charging voltage to the brush charging device. The predetermined voltage is applied to the brush charging device during an inter-non-imaging period (the non-imaging period between formation of two successive images on the electrostatic latent image carrier). As shown in FIG. 1, the brush charging device 210 contacts with the surface of a photoconductor drum (or an electrostatic latent image carrier) 200. Assuming the charge characteristics of the photoconductor drum 200 are of negative polarity, a voltage of -1,100 volts is applied to the brush charging device 210 to charge the surface of the photoconductor drum 200 to -700 volts. Here, the fragments of the toner and the shavings with positive polarity which have failed to be removed by a cleaning device (not shown) are attracted to the brush charging device 210. During the subsequent inter-non-imaging period, the brush charging device 210 receives a temporary applied voltage of 0 volts (grounded), as shown in FIG. 2. The moment the brush charging device 210 reaches 0 volts, the toner fragments and shavings with positive polarity are attracted to the photoconductor drum 200 from the brush charging device. The result is adhesion of the toner fragments and shavings to the photoconductor drum 200 (in region (B) in FIG. 3), as shown in FIG. 3. In FIG. 3, region (A) is the region in which the brush charging device 210 has received the applied 0 volts.
The image forming apparatus described above, however, has the following drawbacks. Usually the photoconductor drum 200 is subjected to surface destaticization by a destaticizer prior to charging, and a 0-volt region is brought into contact with the brush charging device 210, as shown in FIG. 1. Usually, the 0-volt region of the photoconductor drum 200 cannot be charged to -700 (V) rapidly. This provides a charged region 200A extending between the two ends E and S in FIG. 1, with a voltage distribution shown in FIG. 4, wherein the voltage is 0 volts near the position E. Accordingly, as shown in FIG. 2, upon application of 0 volts to the brush charging device 210, the brush charging device 210 and the surface of the photoconductor drum 200 have little potential difference near the position E. Therefore, it is difficult to remove the toner fragments and shavings attached to a region near the position E. This drawback becomes more serious in cases where the brush charging device 210 in the state shown in FIG. 2 receives an additional voltage other than 0 volts (a voltage with the same charging characteristics as the photoconductor drum and having a lower absolute value than its charged voltage).
In addition, as shown in FIG. 3, the backward region (C) of the region (A) of the photoconductor drum 200, being at a voltage of 0 volts, attracts the toner when developed, and produces unwanted black set-solids. In order to prevent this production, the aforementioned prior art is designed to apply a bias voltage with positive polarity (+250 volts) to the developing device while the region (C) is in contact with the developing device. Accordingly, the bias supply for the developing device is required to generate both positively and negatively polarized voltages, and thus it becomes essential to have two separate transformers.